Thanks for the replies! The FastLED works with no "ghost LEDs", but I still have some questions:
When changing leds[some_led_index].r, it actually changes the green value of the led. leds[some_led_index].g changes red. blue works normally. Is there anyone having same problems? Thanks.

Thanks! That worked.
Just out of curiosity: What's the difference between the two?

Run is just for quick testing of your program. It is erased on a reboot. Download will add the program to the flash storage on the device so you can access it from the APP.LIST menu (middle button when the M5stack is booting up - inbeteeen upload and setup).